Silent chains are used as timing chains or chains for auxiliary drives in automobiles, motorcycles and the like. A silent chain is generally formed of a plurality of link plates each having a pair of teeth and pin apertures and pivotably connected to each other via connecting pins inserted into the pin apertures. Also, each of the teeth of the link plates is formed of an inside flank surface and an outside flank surface.
At the time of power transmission through the silent chain, one of the flank surfaces of the teeth of the link plates engages with the teeth of the sprocket, thereby transmitting power from the drive sprocket to the driven sprocket.
In such a silent chain, noise results from a pitch noise generated at the time of engagement of the chain with the sprocket. In order to decrease the pitch noise, a smooth contact of the chain with the sprocket is most desirable without causing a collision between the chain and the sprocket.
Noise also results from chordal action of a chain span generated at the time of engagement of the chain with the sprocket. In order to decrease such chordal action, the chain needs to engage with the sprocket as the chain travels along a straight line.
In summary, when a chain having traveled along the straight line toward the sprocket slides into and engages with the sprocket without colliding with the sprocket and without causing chordal action, a further decrease in noise of the chain is possible.
In another point of view, such movement of the chain and sprocket may be the same motion as a tooth profile formation by a rack cutter. That is, it is possible to replace the linear portion of the chain with a rack cutter and also to replace the sprocket with a gear that is cut by the rack cutter.
From this point of view, the applicant of the present invention proposed a silent chain power transmission device as shown in Japanese patent application laying-open publication No. 8-184838 (JP 8-184838). In such a power transmission device, the protruded flank surface of the teeth on the sprocket engagement side in each of the link rows with the silent chain pulled straight has a shape similar to a portion of the tooth shape of a rack cutter that can cut a sprocket used together with the silent chain. Also, the sprocket tooth surface has a shape that has been cut by the rack cutter.
In this case, a chain having traveled along a straight line toward the sprocket slides into and engages with the sprocket without colliding with the sprocket and without causing chordal action, thus achieving a smooth contact of the link plate with the sprocket.
Thereby, not only occurrence of the pitch noise due to collision of the link plate with the sprocket but also occurrence of the noise due to a chordal action of the chain can be prevented, thus allowing for a further reduction of noise including low frequency noise. Moreover, in this case, since the amount of movement of each of the link rows in a chain span is proportional to the rotational angle of the sprocket, fluctuation of angular velocity can be prevented from occurring.
On the other hand, as a result of long years of diligent study on the noise that occurs during operation of the silent chain power transmission device, it has become clear that noise occurs not only at the onset of engagement of the chain with the sprocket but also at the time of seating (or the end of the engagement) of the chain onto the sprocket after progress of engagement.
Such noise at seating has a high peak value especially in the case of a chain with the engagement mode in which the inside flank surface engages with the sprocket tooth at the onset of engagement and the outside flank surface engages with the sprocket tooth at seating on the sprocket. That is because when the engagement point travels from the inside flank surface to the outside flank surface the load acting on the engagement point abruptly increases.
However, the silent chain described in JP 8-184838 is directed to decreasing noise only at the onset of engagement with the sprocket, and as a result of this, it cannot completely solve the noise issue during operation of the chain including seating.
The applicant of the present invention also proposed the silent chain power transmission device as shown in Japanese patent application laying-open publication No. 2000-65156 (JP 2000-65156). In this silent chain power transmission device, a portion of the sprocket tooth contacting the silent chain at the time of seating on the sprocket tooth has a tooth profile that is an envelope of trajectories described by the inside flank surface of the link plate when the link plate rotates around the connecting pin during the wrapping motion of the silent chain around the sprocket.
When the chain wraps around the sprocket the link plate rotates around the sprocket tooth and the inside flank surface of the link plate seats on the sprocket tooth. In this case, the engagement point of the link plate with the sprocket is located on the inside flank surface at all times from the onset of engagement till the seating. That is, an abrupt variation of the load due to transfer of the engagement point will not occur such as in the case of the chain in which the inside flank surface starts to engage with the sprocket tooth and the outside flank surface finishes engaging with the sprocket tooth to seat on the sprocket. As a result, noise lowers, the load on one sprocket tooth decreases, and the life span of the sprocket improves.
Also, in this case, since the tooth profile of the sprocket tooth is in the shape that is an envelope of trajectories described by the inside flank surface of the link plate during a rotational movement of the link plate, when the chain wraps around the sprocket the inside flank surface of the link plate that rotates around the sprocket tooth slides into and engages with the sprocket tooth without interfering with the sprocket tooth. Thereby, at the time of seating on the sprocket, the occurrence of the pitch noise due to collision of the link plate with the sprocket can be prevented and noise decreases.
In such a way, from the onset of engagement to the seating, noise can be further decreased, and thus noise and oscillation property advances.
On the other hand, in such a silent chain power transmission device, there exists a strong demand to decrease wear to the chain and sprocket. The applicant of the present invention has studied and examined the contact state between the chain and the sprocket during operation.
FIG. 7 illustrates the engaging state of a silent chain with a sprocket. The silent chain has an engagement mode in which the inside flank surface starts to engage with the sprocket tooth and the outside flank surface seats on the sprocket tooth. Link plates 2′ that constitutes a silent chain 1′ has a pair of pin apertures 21′ and teeth 22′. Connecting pins 3′ are inserted into the pin apertures 21′ to pivotably interconnect the adjacent link plates 2′. Each of the teeth 22′ is formed of an inside flank 23′ and an outside flank 24′.
As shown in FIG. 7, a link row L1′ is located at the front row of a linear span of the silent chain 1′ that moves linearly toward a sprocket S′. The inside flank surface 23′ of the link row L1′ is in the state of onset of engagement with a sprocket tooth t′. A link row L2′ is in the state of engagement by a linear motion of the silent chain 1′. A link row L3′ is in the state of engagement by a rotational motion of the link plate 2′ when the silent chain 1′ wraps around the sprocket S′. A link row L4′ is in the state of seating on the sprocket tooth t′ when the engagement has come to an end.
In FIG. 7, a reference mark T′ designates a chain tension, and a reference mark F′ designates a reaction force that the inside flank surface 23′ of the link plate 2′ receives from the sprocket tooth t′ in the state of engagement. Resultant force of a plurality of the reaction forces F′ combined in the direction along the chain tension is balanced by the chain tension T′. As can be clearly seen from FIG. 7, the reaction force F′ that the inside flank surface 23′ of the link plate 2′ receives from the sprocket tooth t′ becomes greatest at the positions of the link rows L2′ and L3′. That is, the reaction force F′ has the maximum value when the silent chain finishes the linear motion to shift to the rotational motion.
On the other hand, with regard to the state of wear to the inside flank surface of the silent chain and the sprocket tooth after a lapse of a certain amount of operating hours, it has been found that an encircled portion A′ in FIG. 7 was worn most badly. The encircled portion A′ corresponds to the position where the inside flank surface 23′ of the link plate 2′ receives the maximum reaction force F′ from the sprocket tooth t′.
Based upon the above-mentioned study and consideration, the applicant of the present invention has concluded that if the link plate of the silent chain does not contact the sprocket tooth at the position where the maximum reaction force occurs at the time of engagement of the silent chain with the sprocket then wear to at least the said position can be prevented.
The present invention has been made in view of the above-mentioned study and consideration, and its object is to provide a silent chain transmission device that can not only decrease noise from the onset of engagement till the seating on the sprocket but also reduce wear.